The present invention relates to a circuit with a photo or optical coupler.
A circuit including a combination of an electric/light converting element and a light/electric converting element is generally called a photo or optical coupler. An equivalent circuit of the photo coupler is shown in FIG. 1A or 1B. In the equivalent circuit shown in FIG. 1A, an input current I.sub.F is converted by a light-emitting diode LD into a corresponding light signal. The light signal is received and converted again into a current signal by a photo diode PD. The converted current signal is amplified by a transistor Q.sub.A. An output current Ic fed from the collector/emitter path is used as an output current of the photo coupler. In the equivalent circuit shown in FIG. 1B, an input current I.sub.F is converted by a light-emitting diode LD into a corresponding light signal. The light signal is received by a photo transistor Q.sub.P. An output current Ic of the photo coupler circuit is taken out from the collector/emitter path of the transistor Q.sub.P. In this case, the output current Ic is given by the following equation: ##EQU1## where K is the factor of proportionality, I.sub.F ' is the input current (reference input current) when the factor of proportionality K is measured, and n is the inclination of the I.sub.F -Ic characteristic curve when it is logarithmically expressed.
The I.sub.F -Ic characteristics of three photo couplers which are commercially available are shown in FIGS. 2A to 2C. In these figures, the abscissa represents an input current I.sub.F, and the ordinate represents an output current Ic. As seen from the characteristic curves, the index n is not constant over the entire range of the input current I.sub.F ; it is approximately "2" in a small current region (I.sub.F &lt;5 mA); in a large current region (I.sub.F &gt;10 mA), n is approximately "1". This fact indicates that the current transfer ratio is nonlinear. A nonlinear current transfer ratio is problematic when the photo coupler circuit is incorporated into a linear circuit, for example, as a current source circuit for gaining an output current exactly corresponding to an input current.
There is known a current source circuit which is designed so as to have an output current proportional to the input voltage. In such a current source circuit, the polarity of the output current with respect to that of the input voltage depends on the type of the current mirror circuit used as an output stage. This implies that the polarity of the output current is determined solely on the type of the current mirror circuit. This greatly restricts the circuit design.
There is further known a voltage/current converting circuit with the above photo coupler designed so as to produce a current corresponding to an input voltage. Because a nonlinear photo coupler is used, the converting circuit can not produce an output current exactly proportional to the input voltage. Also in this converting circuit, the polarity of the output current depends on the current mirror circuit of the output stage. This restricts the design freedom of the circuit, as it does in the current source circuit.